The present invention relates to a process for the preparation of solutions of anionic organic compounds, to the solutions so prepared and to the use of such solutions. In this context, anionic organic compounds are understood to be, especially, dyes and fluorescent whitening agents and also intermediates for the preparation thereof.
In recent years, the use of concentrated aqueous solutions, for example of dyes and fluorescent whitening agents, has gained in importance, that being the case because of the advantages that such solutions have over the corresponding powder forms. The use of solutions avoids the difficulties associated with dust formation and frees the users from the time-consuming, and often difficult, task of dissolving the powder in water. The use of concentrated solutions has also been encouraged by the development of continuous processes for dyeing or whitening paper because it is advantageous in those processes for the solution to be introduced directly Into the hollander or added at some other suitable point in paper manufacture.
In the case of a number of dyes and fluorescent whitening agents, however, formulating concentrated solutions presents difficulties because the concentrated solutions, especially when they still comprise significant amounts of inorganic salts, tend to gel. It is then not practically possible for such gels to be purified and/or desalted by filtering off and washing.
Furthermore, on storage, especially at temperatures below room temperature, there are often formed in the concentrated solutions deposits which either cannot be redissolved at all or can be redissolved only by carrying out additional work. Moreover, if concentrated anionic dye or fluorescent whitening agent solutions are to be suitable as commercial forms, they should, on being diluted to produce the dyebaths, yield clear solutions containing about from 1 to 3% by weight of dye or fluorescent whitening agent without a precipitate, and that should also be the case over a pH range that is as wide as possible.
The present Invention Is based on the problem of providing suitable concentrated solutions of such dyes and fluorescent whitening agents and also intermediates for the preparation thereof in which the mentioned difficulties do not occur.
It has now been found that, by means of the process described hereinbelow, it is possible to prepare, simply and economically, concentrated solutions that excellently meet the demands made. The process represents a simple and economical method for converting anionic organic compounds present in a poorly soluble salt form into a readily soluble form by temporarily converting particular or all acid groups Into the acid form and subsequently neutralising them using suitable bases.
The present invention accordingly relates to a process for the preparation of concentrated solutions or suspensions of anionic organic compounds, which process comprises
a) acidifying an aqueous solution or dispersion of an anionic organic compound that comprises salts and/or impurities, to a pH of 4.5 or less, if the pH is above that value, so that
b) the anionic organic compound becomes insoluble in water and precipitates out in the form of the free acid,
c) bringing the suspension, by means of micro- or ultra-filtration, to a salt content of less than 2% by weight, based on the total weight of the retained material, with
d) optional washing out of the salts with water at a pH of less than 4.5,
e) then optionally washing with water until acid-free, then
f) increasing the concentration so that the content of anionic organic compound is from 5 to 50% by weight, and
g) optionally dissolving the anionic organic compound by addition of a suitable base.
Anionic organic compounds are to be understood as being especially dyes and fluorescent whitening agents and intermediates for the preparation thereof.
Suitable dyes for the process according to the invention are anionic dyes that are stable and insoluble in water at pH values of less than 4.5. Such dyes may belong to any desired class. They are, for example, dyes containing at least one sulfonic acid group and/or carboxylic acid group from the following classes of dyes: metal-free or metal-containing mono-, bis- and poly-azo dyes, pyrazolone, thioxanthone, oxazine, stilbene, formazan, anthraquinone, nitro. methine, triphenylmethane, xanthone, naphthazarine, styryl, azastyryl, naphthoperinone, quinophthalone and phthalocyanine dyes. Such dyes may contain one or more fibre-reactive groups in the molecule.
Preference is given to azo dyes containing at least one sulfo group and, amongst those, especially the so-called azo direct dyes, for example those referred to in The Colour Index, Third Edition, Volume 2 (The Society of Dyers and Colourists, 1971). A further preferred dass is that of the so-called stilbene dyes.
Special preference Is given to dyes that are suitable for the dyeing of paper and, amongst those, especially the dyes of formula 
wherein KK is the radical of a coupling component.
KK is preferably a coupling component of formula 
wherein
Y1 and Y2 are each independently of the other xe2x95x90O, xe2x95x90NH or xe2x95x90Nxe2x80x94C1-C4alkyl,
Y3 is xe2x95x90O, xe2x95x90S, xe2x95x90NR or xe2x95x90Nxe2x80x94CN, R being hydrogen or C1-C4alkyl, and
R1 and R2 are each independently of the other hydrogen, unsubstituted or substituted alkyl or unsubstituted or substituted phenyl.
In formula (2) above, only one tautomeric form is indicated for the coupling component, but the formula is intended also to encompass the other tautomeric forms.
When R1 and/or R2 is/are an unsubstituted or substituted alkyl group, it is to be understood as being, for example, methyl, ethyl, n- or iso-propyl, n-, sec- or tert-butyl, straight-chain or branched pentyl or hexyl or cyclohexyl; the said radicals may be mono- or poly-substituted, for example by OH, C1-C4alkoxy or by C1-C4hydroxyalkoxy.
Examples of suitable substituted alkyl radicals are: methoxymethyl, ethoxymethyl, ethoxy-ethyl, ethoxypropyl, n-propoxymethyl, butoxyethyl and 2-hydroxyethoxypentyl.
When R1 or R2 is unsubstituted or substituted phenyl, the lafter may be mono- or poly-substituted, for example by C1-C4alkyl, C1-C4alkoxy, halogen, e.g. fluorine, chlorine or bromine, or by nitro.
R1 and R2 are preferably hydrogen or C1-C4alkyl.
Y1 and Y2 are preferably xe2x95x90O or xe2x95x90NH; furthermore, Y1 and Y2 are preferably the same.
Y3 is preferably xe2x95x90O, xe2x95x90S, xe2x95x90NH or xe2x95x90Nxe2x80x94CN, especially xe2x95x90NH.
The dyes of formula (1) are known or can be synthesised in a manner known per se.
The stibene dyes are complex dye mixtures which result from the condensation of 4-nitro-toluene-2-sulfonic acid with itself or with other aromatic compounds. Their structure is defined by the mode of preparation. Suitable stilbene dyes are, for example, those described in The Colour Index, Third Edition, Volume 4 (The Society of Dyers and Colourists, 1971) under the constitution numbers from 40.000 to 40.510.
Suitable dyes for the process according to the invention are preferably Direct Yellow 11 and its derivatives Direct Yellow 6 and Direct Orange 15, which derivatives are obtainable by means of reductive sub-steps additionally incorporated into the synthesis.
Suitable fluorescent whitening agents for the process according to the invention are sulfo- and/or carboxy-group-containing whitening agents of various classes, for example bis(triazinylamino)stilbenes, bis(triazolyl)stilbenes, bis(styryl)biphenyls and bis(benzofuranyl)biphenyls, bis(benzoxalyl) derivatives, bis(benzimidazolyl) derivatives, coumarin derivatives and pyrazoline derivatives.
For example, the process according to the invention is suitable for the preparation of concetrgted solutions of the following fluorescent whitening agents: 
Suitable intermediates for the process according to the invention are especially anionic intermediates that are used for the synthesis of dyes or of fluorescent whitening agents.
They are, especially, aromatic sulfonic acids that also carry one or more further substituents, for example amino, nitro, alkyl or hydroxy.
Especially suitable intermediates are, for example, 2-amino-5-hydroxynaphthalene-7-sulfonic acid, 4-aminotoluene-2-sulfonic acid, dehydroparathiotoluidinesulfonic acid, 4,4xe2x80x2-diamino-stilbene-2,2xe2x80x2-disulfonic acid, 4,4xe2x80x2-dinitrostilbene-2,2xe2x80x2-disulfonic acid, 4,4xe2x80x2-diamino-diphenyl-amine-2-sulfonic acid and 4-nitrotoluene-2-sulfonic acid.
The process according to the invention is carried out in particular as follows:
The process usually starts from an aqueous synthesis solution or suspension that, besides the anionic organic compound, also comprises greater or lesser amounts of starting materials, secondary products, salts or other impurities. If, however, the anionic organic compound is present in solid form or in the form of a slurry or paste, it is first dispersed in water so that an aqueous suspension or solution is obtained.
If the anionic organic compound is already present therein in the form of the free acid, the micro- or ultra-filtration is carried out directly thereafter, whereas if it is present in salt form, the first step of the process according to the invention is to convert the salt into the free acid.
In the case of compounds containing a plurality of sulfo groups it is sometimes advantageous to carry out the conversion into the free acid in a plurality of steps at different pH values and/or temperatures or to convert only particular sulfo groups into the free acid.
For preparation of the free acid, an aqueous solution or dispersion of the anionic organic compound, which comprises salts and/or other impurities, is acidified to a pH of 4.5 or less and is stirred or mixed until the anionic organic compound has been virtually completely converted into the free acid and is therefore insoluble in water and precipitates out That is carried out preferably by adding a strong inorganic acid, for example hydrochloric acid or sulfuric acid, until the desired pH has been obtained. The conversion is advantageously carried out at a temperature of from 15 to 140xc2x0 C., especially from 20 to 95xc2x0 C.
The optimum pH, the temperature, the concentration and the duration of mixing must be matched to the anionic organic compound and the desired degree of conversion. The optimum conditions can readily be determined by means of appropriate tests.
In the case of anionic organic compounds that are difficult to convert it can be useful first to subject the solution or suspension to partial desalting and only then to carry out conversion into the free acid. That can be done, for example, by means of nanofiltration or intermediate isolation of the anionic organic compound. In addition, special synthesis techniques for generating low-salt synthesis solutions can be used, for example simultaneous diazotisation and coupling. It is also possible to wash an anionic organic compound that has been only partially converted into the free acid, until it has a low salt content and then to add acid again and, optionally at elevated temperature, to carry out stirring or mixing.
Washing and conversion into the free acid can also be carried out in continuous succession by circulating the suspension through a micro- or ultra-filtration unit which is connected in series to a reactor for converting into the free add and, optionally, heating.
In the process according to the invention microfiltration is preferably carried out. However, ultrafiltration can also be used. Because of the relatively fine membranes, the latter is suitable especially for compounds having an amorphous structure. However, the performance is often worse.
Micro- or ultra-filtration is carried out by generally known methods that are customary per se using known membranes. The membranes may consist of acid-resistant organic or inorganic material. Ceramic membranes are especially suitablexe2x80x94in the case of microfiltration especially those having a pore size of from 20 to 1000 nm, more especially from 100 to 800 nm, and in the case of ultraflftration especially those having a pore size of from 1 to 20 nm.
The temperature during micro- or ultra-filtration is approximately at from room temperature to about 95xc2x0 C. preferably from 50 to 85xc2x0 C. The pressure is dependent, inter alia, on the nature of the membrane, but is usually from 1.5 to 10 bar, preferably from 3 to 6 bar.
Washing and increasing the concentration by means of micro- or ultra-filtration is carried out until the desired salt content and the desired concentration of anionic organic compound are obtained. Normally, a content of inorganic salts of less than 2% by weight, preferably less than 0.5% by weight, based on the total weight of the suspension, is sought.
The content of anionic organic compound after micro- or ultra-filtration is preferably from 5 to 50% by weight, especially from 10 to 40% by weight, based on the total weight of the suspension.
After micro- or ultra-filtration, any desired base can be added to the low-salt or salt-free suspension obtained, in order to obtain readily soluble salts of the anionic organic compounds with any desired cations. Suitable bases are, for example, LiOH, NH4OH, or organic amines, e.g. a C4-C12trialkylamine, C4-C12dialkylamine, C2-C15alkanolamine or polyglycol amine. Preference is given to the use of LiOH, NH4OH or an alkanolamine.
The solutions of dye or whitening agent obtained may be used directly in that form or optionally after dilution. They may also, however, in customary manner, be dried and used in the form of powders or granules.